For Gulbransen parts and service - Daves'electronics here in the US
Gulbransen (All Pre-1986 Models: Parts, Service and Owners Manuals and Tech Support)
105A East Penn St.
Hoopeston, IL 60942
tech support; George Reynertson email@example.com
You may have to send the entire PS unit over there for servicing, or, if you are certain it is the transformer, maybe they have one they can send over to you. Be sure to mention it is the 240v model. THey also sell the service manuals/schematics so this may also help.
You can write directly to George (he is widely known to be the expert onthe earlyGulbransens) but don't ask for him directly if you call the shop..
Transformers rarely fail for no reason. If there was no known cause for the failure (some catastrophic damage) then the entire PS unit should be looked at very closely. Bad power supply filter capacitors can take a transformer down so you should replace those is you haven't done so already. I would be curious to know what the syptoms were before the organ stopped working. If the user reported that there was loud humming then the filter caps probably failed and took down the transformer.
Looking at my schematic (remember mine is the US version)- The power supply unit, looking at it as a whole, should produce the following voltages for the organ: +17, +20, +25, -35, +35DC, as well as 13v AC. The +20 and +25 DC are regulated; the rest are not from what I can tell (I'm certainly no expert!). Part numbers for your reference PS assembly: 413014. Rectifier board 413011, Voltage regulator board: 413003. There are several large value filter cap values used in this unit:5000/32, 4500/20, 2500/20 among them.
I would contact Dave's and see if the replacement transformer is available and if so you should also replace all the filter caps just to be sure.